This invention relates in general to electric arc gas heaters and more particularly to an electric arc gas heater in which the arc is maintained at a substantially fixed length in a region of high pressure to effect a high voltage gradient throughout the length of the arc and thereby transfer maximum energy to the gas stream.
One of the more practical procedures for testing the durability of materials and configurations at high temperatures and velocities, is to place the material or configuration, or at least a scale model of the configuration, in a high velocity-high temperature airstream created by an electric arc gas heater. Basically, such heaters maintain an arc in the air reservoir so that the high energy of the arc is transmitted to the air in the reservoir and elevates the temperature thereof. The heated air is then discharged through a nozzle against the object or configuration well beyond the downstream terminus of the arc.
In some arc heaters, the arc is generally fixed in length, and in these heaters the upstream terminus of the arc is normally a button-like electrode. While fixed length arc heaters deliver energy uniformly to the airstream, the button-like electrode erodes rapidly and hence these heaters are not very durable.
In other arc heaters, the arc is maintained between the interior surfaces of two axially aligned tubular electrodes and as a result the arc is of "natural length." In natural length arc heaters the length of the arc continually varies and likewise so does the energy delivered to the airstream.
U.S. Pat. No. 3,590,219 discloses an electric arc gas heater in which the rear terminus of the arc attaches to a tubular electrode while the front terminus attaches to an upstream electrode along the divergent portion of the nozzle. The throat of this heater is of constant diameter and quite long, and the arc extends the entire length of the throat. Due to the acceleration of the flow through the throat, a substantial pressure gradient exists within it, that is the pressure at the upstream end of the throat is substantially higher than the pressure at the downstream end. Since it is known that the voltage gradient (volts/inch) drops off in proportion to the square root of the pressure (lbs./inch.sup.2), the voltage gradient at the downstream end of the arc is substantially less than the voltage gradient at the upstream end and as a result considerably more power (volts .times. amperes) is delivered to the airstream in the upstream portion of the throat.